At first the U. S opposed annexing Texas because it was concerned with -

I think it's because they wished to avoid a war with Mexico, whose government refused to acknowledge the sovereignty of its rebellious northern province.

Explanation:

google helped

abraham lincoln, according to anyone who was connected with agriculture, represented the worst of northern politicians. shortly after lincoln was elected, south carolina seceded from the union. they along with other southern states, retained their anger. in a way, it was their own fault: the democrats were split in the south. some wanted slavery to be extended in other states that were about to join the union and some didn't want it badly enough. lincoln had a solid backing from his party: the republicans were united in not wanting slavery to be expanded.